An optical communication system performs communications by transmitting main signal light through an optical fiber. In the optical communication system, the power of WDM main signal light transmitted from an optical transmitter decreases due to transmission loss during the time when it reaches an optical receiver through the optical fiber. If the power of the main signal light when it reaches the optical receiver is not higher than a predetermined value, the optical receiver cannot read the main signal light. This causes an error and the optical communication not successfully be carried out. One approach to avoid this problem is to provide an optical amplifier between the optical transmitter and the optical receiver and amplify the power of the main signal light with the optical amplifier to compensate for the transmission loss of the main signal light. By providing the optical amplifier, a main signal light with a predetermined power or higher can be input into the optical receiver.
As the optical amplifier, a Raman amplifier is available. The Raman amplifier supplies a pump light to an optical fiber, which is an amplifying medium through which a main signal light propagates, and amplifies the main signal light by using a nonlinear phenomenon called “stimulated Raman scattering”. FIG. 1 is a block diagram of a schematic configuration of a conventional Raman amplifier (see, for example, Patent literature 1). The Raman amplifier transmits the WDM main signal light in which the n-number of main signal lights having wavelengths λs1 to λsn [μm] (λs1<λsn) are wavelength-division multiplexed. The Raman amplifier includes a silica-based glass optical fiber (e.g., single-mode optical fiber (SMF)) 101 for providing Raman amplification, and two pump light sources 103a and 103b that generate pump lights having different wavelengths λp1 and λp2 [μm] (λp1<λp2). The Raman amplifier also includes a first photocoupler 102a that couples the pump lights generated in the pump light sources 103a and 103b, a second photocoupler 102b that guides the pump lights coupled, to the silica-based glass optical fiber 101, and an isolator 104 that prevents backflow of the WDM main signal light Raman-amplified in the silica-based glass optical fiber 101. In this example, the two pump light sources 103a and 103b are used to flatten the waveform characteristics of gain of the main signal light Raman-amplified.
The operation of the Raman amplifier is explained below. The WDM main signal light is introduced into the silica-based glass optical fiber 101 that is a transmission path and a Raman amplifying medium. On the other hand, the two pump lights having different wavelengths emitted from the two pump light sources 103a and 103b are coupled by the first photocoupler 102a, and the pump lights coupled are introduced into the silica-based glass optical fiber 101 in a reverse direction to a direction of travel of the WDM main signal light by the second photocoupler 102b. The pump light causes an optical amplifying action due to the phenomenon of stimulated Raman scattering in the silica-based glass optical fiber 101 that is an amplifying medium. The WDM main signal light is thereby Raman-amplified in the silica-based glass optical fiber 101, and is output.
FIG. 2 is a spectrum of the pump light/signal light and the Raman gain bandwidth in the silica-based glass optical fiber of the conventional Raman amplifier. The Raman gain bandwidths for the pump light wavelengths λp1 and λp2 [μm] in the Raman amplification are formed as a wavelength range on the longer wavelength side by about 0.1 [μm] from the respective pump light wavelengths. The wavelength range covers almost both the wavelengths of the two pump lights. For example, a Raman gain bandwidth for the pump light having the wavelength λp1 [μm] is in a range indicated by a solid line L1, and a Raman gain bandwidth for the pump light having the wavelength λp2 [μm] is in a range indicated by a broken line L2. The Raman gain bandwidth achieved by being pumped by the two pump lights having the wavelengths λp1 and λp2 [μm] is in a range indicated by a solid line L0 in which the Raman gain bandwidths corresponding to the respective wavelengths of the pump lights are combined.
As shown in FIG. 2, the main signal light positioned on the shortest wavelength side of the WDM main signal light is pumped mainly by the pump light having the short wavelength λp1, while the main signal light positioned on the longest wavelength side of the WDM main signal light is pumped mainly by the pump light having the long wavelength λp2. Moreover, a wavelength at around the center of the WDM main signal light is pumped by the pump lights having the two wavelengths λp1 and λp2, and is thereby Raman-amplified. In other words, how the two pump lights contribute to individual main signal lights of the WDM main signal light depends on the wavelengths of the pump lights. Therefore, by adequately setting the wavelengths λp1 and λp2 of the pump lights to control the power of the pump lights from the pump light sources 103a and 103b, the Raman gain bandwidth can be made widened so as to cover the wavelengths λs1 to λsn of the WDM main signal light. As a result, the loss of the silica-based glass optical fiber 101 can be compensated for over a broader bandwidth in a required signal transmission bandwidth.
In the optical communication system, an optical amplification transmission system, which compensates for the loss of signal light using the Raman amplifier, is set in one transmission section and a many such transmission sections are cascade-connected. This structure allows construction of a long-distance optical communication system. In the long-distance optical communication system also, the wavelength of each pump light and the power of the pump light are controlled in each transmission section to keep a Raman gain bandwidth to a broad bandwidth in each transmission section, which makes it possible to carry out long-distance transmission of WDM main signal light having a broad bandwidth.
In the optical communication system that amplifies and repeats the main signal light between an optical transmitter and an optical receiver, in order to transmit a control signal for remotely controlling an optical repeater that forms the optical communication system, a control signal with network management information thereon needs to be transmitted in an optical network thereof, different from the main signal light. There is a known structure for this. In this structure, the intensity of a digital modulation signal light is further modulated with a control signal having a comparatively low frequency to allow it to propagate through the silica-based glass optical fiber. For example, a control signal superimposing device is disclosed. The control signal superimposing device superimposes a control signal in the optical transmission system on the main signal light by using the fact that the intensity of a main signal light Raman-amplified by pump light intensity-modulated with the control signal also fluctuates according to the control signal (see, for example, Patent literature 2).
Patent Literature 1:
Japanese Patent Application Laid Open No. HEI 10-73852 (pages 2 to 3, FIG. 2 and FIG. 4)
Patent Literature 2:
Japanese Patent Application Laid Open No. HEI 11-344732 (pages 3 to 4, FIG. 1 and FIG. 2)
However, in the case of the optical amplifier disclosed in the Patent literature 1 as a conventional technology, it is mandatory to control the power of individual pump lights so that the level of the WDM main signal light Raman-amplified becomes uniform. In this case, as shown in FIG. 2, a main signal light having a wavelength is pumped by pump lights having a plurality of wavelengths, and moreover, the pumping by the pump lights is different in respective main signal lights, which makes it difficult to control such power. If there is loss of a plurality of silica-based glass optical fibers and there are fluctuations in Raman gain coefficient in particular, or if the loss of the silica-based glass optical fibers fluctuates, it is not easy to control the power of individual pump lights so that the level of the WDM main signal light Raman-amplified becomes uniform.
On the other hand, the control signal superimposing device disclosed in the Patent literature 2 has an object to keep constant the modulated intensity of a main signal light, that is, to stably keep the modulated level of a control signal component. The object is achieved by superimposing a control signal transmitted in the optical transmission system or an optical network on the main signal light, and preventing a control signal component superimposed on the main signal light from being in over-modulation. Therefore, the control signal superimposing device is not related to a technology that controls the power of individual pump lights so that the level of the main signal light Raman-amplified becomes uniform, in other words, the Raman amplification gain becomes a predetermined value.
The present invention has been achieved to solve the problems, and it is an object of the present invention to obtain an optical amplifier capable of controlling Raman gain so that the level of a WDM main signal light Raman-amplified becomes uniform, and an optical communication system using the optical amplifier.